In the course of a surgical operation the loss of blood suffered by the patient through the incision must be carefully monitored so that a corresponding volume of blood may be transfused into the patient to avoid loss of blood pressure. For this reason the blood flowing from the incision is carefully gathered by a suction device and accumulated in calibrated containers. The estimation of actual blood loss is complicated by the fact that the area of the incision is continuously irrigated by a cleaning solution which is also drawn by the suction device and ends up mixed up with the blood in the waste containers.
It is, therefore, necessary to also monitor the amount of solution which is being used in the course of the operation. Subtracting the amount of solution used from the volume of mixture gathered in the waste container does not provide an accurate figure of the blood loss due to the fact that a large amount of solution is sprayed around the area of the incision and becomes absorbed by sponges, linen etc.
During a lengthy surgical intervention, the pre-surgical and post-surgical weighing of the solution and mixture can only provide an after-the-fact estimate of the total blood loss, when a continuous or at least periodical estimate would be preferable in order to adjust the amount of transfusion to the actual blood loss and thus avoid sudden, and sometime fatal loss of the patient's blood pressure.